It would be the same as your 2nd matrix with the right four columns negated.

doug

Stefan Brauns wrote:

Hi all,

I am trying to examine the interaction of 2 factors (2 x 4 groups) and have difficulties to find the right contrast matrix for mri_glmfit.

We have patients and controls at four different aquisition sites and want to know if the changes (i.e. thickness) between patients and controls are equal in every site.

so we have 8 groups:

patientssite1 patientssite2 patientssite3 patientssite4 controlssite1 controlssite2 controlssite3 controlssite4

the contrast for differences between patients and controls (T-Test) would be:

1/4 1/4 1/4 1/4 -1/4 -1/4 -1/4 -1/4

the contrast to investigate if the observed measured value is equal in all sites (F-Test) would be:

1/2 -1/2 0 0 1/2 -1/2 0 0 0 1/2 -1/2 0 0 1/2 -1/2 0 0 0 1/2 -1/2 0 0 1/2 -1/2

How would the contrast matrix look like if I want to test if there is an overall interaction between site and diagnosis, i.e. does the effect of diagnosis vary by site (F-Test)?

Many thanks, Stefan

-- Stefan Brauns

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